TiO2 photocatalysis has been extensively studied for the removal of organic contaminants in water or air (1-11). Mixed-phase TiO2-based composites tend to show higher photoreactivity in comparison to pure-phase materials, as exhibited by commercial Degussa P25, due to the formation of solid-solid interfaces that facilitate charge transfer and spatial separation, reduced electron-hole recombination, and interfacial defect sites that act as catalytic “hot spots” (2-7).
Recently, efforts to combine TiO2 with activated carbon (12, 13) or carbon nanotubes (CNTs) in simple mixtures or as nanocomposite materials sought to create more highly reactive photocatalysts (14-18). The synthesis of TiO2/CNT nanocomposites using sol-gel, chemical vapor deposition (CVD), and physical vapor deposition (PVD) methods has been reported (17, 19-21). The addition of multi-walled CNTs (MWCNTs), prepared by a catalytic deposition method, to a slurry of Degussa P25 improved color removal in an aqueous solution of azo-dyes under ultraviolet light (16). The researchers, however, did not fully explain how a simple mixture of TiO2 and MWCNTs would interact to accelerate the decay of the dye. MWCNTs coated with a uniform layer of anatase TiO2 using a modified sol-gel process displayed a higher rate of phenol degradation in water than either of the components alone or in simple mixtures (14). The authors proposed that the MWCNTs served as a catalyst support increasing the reactive surface area of the TiO2 in suspension and suggested that intimate interphase contact between the TiO2 and MWCNTs has other synergistic photochemical effects. MWCNTs coated with anatase using a sol-gel method showed high photocatalytic inactivation of bacterial endospores dispersed in water (15). The authors indicated that the MWCNTs not only provided a large surface area support for the titania catalyst, but also stabilized the charge separation by trapping the electrons transferred from TiO2, thereby hindering charge recombination.
Previous work, then, illustrates that there are a variety of ways to combine TiO2 and CNTs to create a photocatalyst. The structure of the CNTs in combination with TiO2, however, is not clear. For example, TiO2 nanoparticles and CNTs may be combined in a random composite, as shown in FIG. 1(a), or CNTs may be coated with TiO2 nanoparticles, as shown in FIG. 1(b). In both cases, CNTs disperse the TiO2, increasing the reactive surface area. Furthermore, given the conducting and semiconducting properties of CNTs, it is also possible that photoexcited electrons from TiO2 may be transferred to the CNTs to hinder recombination and enhance oxidative reactivity.